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              <div class='slide'>
  <p>A <code>4:3</code> box that always has at least one dimension covering the entire viewport. If you wish to change this ratio, you can do so by changing the value of the <code>$a</code> and <code>$b</code> variables in the SCSS. Note that both need to be integers!</p>
  <p>Resize the viewport to view the effect.</p>
              @mixin proportional-box($a: 1, $b: $a) {
	position: absolute;
  top: 0; left: calc(50vw - #{$a/$b/2*100vh});
  width: $a/$b*100vh; height: 100vh;
  @media (max-aspect-ratio: #{$a}/#{$b}) {
    top: calc(50vh - #{$b/$a/2*100vw}); left: 0;
    width: 100vw; height: $b/$a*100vw;

html { background: #f9f9f9; }

body { margin: 0; }

.slide {
  /* relevant part */
  @include proportional-box(4, 3);
  /* non-essential styles */
  overflow: hidden;
  box-sizing: border-box;
  padding: .5em;
  background: #e53b2c;
	color: #f9f9f9;
  font: 4.5vmin sans-serif;
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